1. Field of the Invention
The present invention relates to an electron beam excited plasma system for use in the manufacture of semiconductor devices or other types of devices.
2. Description of the Related Art
In an electron beam excited plasma system (hereinafter referred to as an EBEP system), a plasma is generated by exciting a reaction gas by means of electron beams. The plasma, thus generated, is utilized when a semiconductor wafer is subjected to etching, doping, oxide film formation or cleaning. Typical types of conventional EBEP systems are disclosed, for example, in Jpn. Pat. Appln. KOKAI Publications No. 63-190299, No. 1-105539, and No. 1-105540. The first KOKAI Publication No. 63-190299 discloses an EBEP system wherein a pair of electrodes are secured, with a spacer interposed therebetween. The other two KOKAI Publications disclose an EBEP system which comprises a deflecting electrode used for diffusing (or spreading) electron beams.
In these conventional EBEP systems, an electric discharge is produced between the tip end of a cathode and an anode or an intermediate electrode in the presence of argon gas, and electrons are extracted from the plasma generated by the electric discharge. The cathode is located on the central axis of the EBEP system, like the other electrodes. The cathode is made up of an initial discharge section formed of a high melting point metal, and a thermoelectron emission section formed of a lanthanum compound.
In a conventional EBEP system, the cathode is bombarded with ions at the time of electric discharge, resulting in the occurrence of sputtering. The metal atoms emitted during the sputtering (which will be hereinafter referred to as sputtering substances) may enter the reaction chamber and contaminate semiconductor wafers under treatment. Therefore, the convention EBEP system does not necessarily satisfy its requirements, such as fine working, little system damage, and high-speed plasma processing.
In the conventional EBEP system, the initial discharge section and the thermoelectron emission section are located at the tip end of the cathode and face the intermediate electrode. Due to this structure, the lanthanum compound of the thermoelectron emission section is chipped or partly lost when it is subjected to an impact caused by the bombardment of ions at the time (start up) of the actuation of the system.
The lanthanum compound cannot continuously emit thermoelectrons in a stable manner unless its temperature exceeds 1,000.degree. C. This being so, the plasma generated by electric discharge is not very stable, and may be lost depending upon the conditions of the electric discharge. In addition, since a long time is required before the plasma becomes stable, the life of the ignition electrode is inevitably short.